Spring Reverb Tanks Explained and Compared

SPRING
REVERB TANKS EXPLAINED & COMPARED

The
Reverberation Effect3

A
listener standing some distance from a sound source will perceive
sound that is actually a combination of direct sound and indirect
sound that has been reflected from the boundaries of the listening
area. The reflections are referred to as reverberation. Reverb can
enhance the perceived sound from a source by adding depth, color and
liveliness.

Imagine
you are inside a large hall and you clap your hands once. The length
of time required for the arrival of the very first reflections is
called the delay
time (usually on the order of tens of milliseconds, e.g. 33 ms) and is
related to the volume of the room (or distance of the reflective
surfaces from the listener). The number and density of reflections
increases rapidly with time and they become cluttered while
simultaneously decreasing in level until they are no longer audible.
The length of time required for a sound to decrease in level by 60 dB
is called the decay
time (usually on the order of a few seconds, e.g. 3 s) and is related to
the acoustical properties of the reflective surfaces in the listening
area. For example, poured concrete walls will reflect more (absorb
less) acoustic energy than drywall.

Electro-Mechanical
Reverberation Devices: The Reverb Tank

Historical
use in Musical Instruments

Laurens
Hammond of Illinois popularized the use of artificial reverberation
devices through his church organs in the 1940's and 1950's. The
early (pre-B-3®)
Hammond®organs were sold to churches on the principle that organ music is
greatly enhanced by reverberation, but the minister's speech in the
church is hampered by reverberation. Therefore, churches were
designed to be acoustically dead, and the Hammond®organ had to have its own artificial reverberation."1 [Reverberation] made its debut in the Fender®line as a separate item, using a spring [unit] bought from Hammond®,
[in] 1961. It was first incorporated in a Fender®amplifier with the Vibroverb®of 1963 and then spread widely throughout the amp line, just as
vibrato/tremolo had before it."2

Each
transducer consists of a coil centered around a magnetic lamination
and small cylindrical magnets centered in the air gap of the
lamination.

Transmission
Springs

These
components are mounted on an inner aluminum channel, which is
connected by four small support springs to an outer steel chassis (or
channel).

An
electrical signal applied to the input transducer coil generates an
alternating magnetic field which moves the transducer magnets. The
magnets are mechanically coupled to transmission springs. The signal
is reflected back and forth through the transmission springs with an
amount of delay determined by each spring's diameter, wire gauge
and length. The moving magnets of the output transducer generate an
alternating magnetic field which induces an electrical signal in the output transducer coil.

The
use of multiple transmission springs helps to improve the reverb
characteristics. A listener in a large hall with natural
reverberation is not usually standing the same distance from each
reflective surface. Naturally, there will be reflections from
different surfaces having different delay times. The use of multiple
transmission springs with different delay times serves to simulate a
more natural ambiance, as well as improving the overall frequency
response because one spring's response will fill voids or holes in
the other spring's response.

Vintage
Accutronics® specs list the following delay
times per spring:

Decay
time should be selected to suit the application. The same reverberation
decay time that enhances and adds liveliness to the sound of the
guitar can make speech unintelligible.

General decay
time suggestions traditionally used for specific instruments:

Reverb
tanks are supplied in a variety of input and output impedances
(measured at 1kHz) to allow for flexibility in designing drive and
recovery circuits. The input and output transducers can be
characterized as essentially inductive, with impedance rising with
increasing frequency (inductive reactance). When replacing the
reverb tank in an existing amplifier design, it is important to match
the original tank's input and output impedances as closely as
possible. The reverb effect will sound poor or inaudible if the
impedances are not matched.

Because
DC resistance can be easily measured with an ohm meter (and Impedance
@ 1kHz cannot), it is sometimes useful to estimate the impedance by
way of DC resistance.

Because
reverb tanks are electro-mechanical devices, their performance is
affected by how they are mounted.

Mechanical
mounting considerations:

Weight
of springs and displacement of transducer magnets along the air gap

Isolation
from vibrating surfaces

Mechanical
feedback through tight cable connections

Mounting
Planes & Magnet Displacement

The
ideal mounting plane for reverb tanks is one that allows the weight
of the transmission springs to keep the transducer magnets centered
along the air gap and not toward the lamination. The reverb tank
mounting plane that results in this ideal transducer orientation is
referred to as Vertical
Connectors Up.

A
simplified cross-sectional side view of a reverb tank to illustrate
transducer orientation inside the tank in the Vertical
Connectors Up mounting plane.

If
the reverb tank cannot be mounted in the ideal vertical connectors
up plane, the tank should be chosen with magnets that have been
factory adjusted to be centered in the air gap for that specific
mounting plane.

A
simplified cross-sectional side view of a reverb tank: Horizontal
Open Side Downmounting plane.

Mechanical
Isolation:

The
reverb tank should be isolated from vibrating surfaces as much as
possible. Avoid mounting the outer channel of the reverb tank
directly to the mounting surface by using grommets, rubber standoffs,
reverb tank bag and liner or other products designed for mechanical
isolation.

Avoid
mounting on cabinet members that would tend to act as sounding
boards. A small dimensioned rigidly supported surface is best.

Cable
Connections:

Allow
for slack in cables attached to the reverb tank to prevent forming
mechanical feedback paths.

Electrical
mounting considerations include:

External
Magnetic Fields

Even
though the transducers are shielded by the outer steel channel, the
output transducer end in particular should be kept away from
transformer fields. The effectiveness of the shield varies with its
orientation in an external magnetic field.

In
order to suit any grounding scheme, reverb tank connectors come in
all combinations of input and output insulated and non-insulated
phono jacks (a.k.a. RCA jack). A non-insulated phono jack is one
whose outer shell is grounded to the outer steel channel (chassis) of
the reverb tank. Vintage Accutronics® specs recommend insulating both input and output connectors and
grounding the tank chassis separately.

If
the amplifier's connection to the reverb tank's phono jack shell
is not at ground potential, it is important that the tank be chosen
with an insulated connector at that connection point. If the amp's
connection to the reverb tank's phono jack shell is at ground
potential, either insulated or grounded connector may be used at that
connection point.

Drive
& Recovery Circuits

These
schematics are examples of drive and recovery circuits from popular
guitar amplifiers. There are many varieties of tube and solid state
drive and recovery circuits used in guitar amplification resulting in
different input and output impedances. It is important to match the
original tank input and output impedance as closely as possible in
order for the reverb effect to work properly.

Drive
Circuit Design Considerations3:

Use
an input high-pass filter to compensate for the input coil's
inductive reactance. It is recommended that the driver be a current
or voltage source with an output rising 6 dB/octave with increasing
frequency.

Drive
the input coil as hard as possible without overdriving (exceeding
core saturation).

Avoid
DC currents through the coil for maximum headroom before core
saturation.

In
general, a current source is equivalent to a voltage source with a
resistor in series.

In
practice, the resistor should have a value greater than 5 times the
1kHz impedance of the input transducer.

The
voltage required will be:

Recovery
Circuit Design Considerations3:

The
output signal from the tank should be about 1 to 5 mV. Use a preamp
circuit with flat frequency response for recovery.

Use
an output high-pass filter with a 50 Hz to 100 Hz cutoff to lessen
the effects of rumble when the unit is mechanically shocked.

Accutronics®Part Numbering System

Vintage
Accutronics® part numbers consist of a seven character alpha-numeric code with
each character of the part number representing a specification.

A
Comparison of Reverb Tank Brands

For
many years, Accutronics®(a Hammond®Corporation Company) designed and built spring reverberation units in
the United States. Today, most reverb tanks are built in Korea and
China. The following is a comparison of current made reverb tanks,
as well as the Accutronics®(made in USA) tanks which are no longer in production.

Physical
Comparison:

Accutronics®(USA) €“ no longer in production

These
tanks are the industry standard. They were marked as made in the
USA.

Phono
jack shell is grounded to reverb tank outer channel by a tab and
self tapping screw driven into the chassis

3
spring units have transmission springs laid out in two planes

Phono
jack terminals connect to transducer coils via direct wire taps

Sound
Characteristic Comparison:

Here
is a comparison between the four different reverb tank brands
discussed previously. In comparing the sound characteristics of
reverb tanks it is important to remember that the amplifier plays a
major role in how the reverberation effect will sound. (i.e. The
same reverb tank will sound different depending on the amp).

As
was discussed in detail on p. 3, the reverberation effect can be
thought of as being composed of two parts:

1)
Early reflections €“ shape the listener's conception of room size

2)
Cluttered reflections €“ convey the liveliness of a room

In
this comparison we have created two categories to compare the early
reflections and cluttered decaying reflections of each tank. The
same guitar (Mexican Fender® Stratocaster®)
and amp (Mesa Boogie®Mark II-C®)
were used with each tank. A solid chord chop was played as the
easiest way to compare these two characteristics.

Echois used to describe the initial reflections. The first reflections
arrive about as quickly as the blink of an eye, but they can be heard
as being pronounced,
moderate orsubtle.

Flutteris
used to describe the cluttered reflections while they decay over the
matter of a few seconds. As the cluttered waves blend into one
another a fluttering sound can be heard as rapid,
moderate orslow.

Three
Springs vs. Two Springs:

Any
amp using a two spring tank may have its tank replaced by a three
spring tank of the same impedance code (for example, 4AB3C1B vs.9AB3C1B) and vice versa. This will also change the sound
characteristics of the reverberation effect. Three spring tanks have
a more smoothed out flutter with a bigger, fuller sounding reverb
effect and more lows. Two spring tanks have more flutter and grit,
they also seem to capture that vintage 1960's vibe better.